Answer: It sounds like they aren't receiving sufficient water. For mature trees, water should soak 3 feet deep; for newly planted trees, about 2 to 2.5, depending on the size/depth of the rootball when it was planted.
Use a soil probe (any long, pointed piece of metal or wood to poke into the soil) to check how far water has penetrated. The probe moves easily through moist soil, but stops when it hits hard dry soil. There are numerous variables involved for watering schedules, such as type of soil, how fast or slow it drains, sun and wind exposure at your site, temperature, age and condition of the plants and much more. Use the information above to determine how moist the soil is before automatically applying more water. If you use a drip system, it's essential that you allow it to run long enough for water to penetrate the appropriate depth. Depending on the size emitters, soil type, etc. this might take several or many hours. For example, an emitter that puts out one gallon per hour would only put a quart of water on the ground in 15 minutes. If you use a hose or bubbler, the concept is still the same.
Southwestern soil and water both contain salts, which can accumulate in the root zone over time. This salt buildup forms where the water stops penetrating. Short periods of watering cause salts to build up in the top layers of soil and damage or kill your plant. Salt burn shows up as yellowing, browning and eventually death. Deep watering?or leaching?prevents this by flushing the salts past the root zone. Always water slowly, deeply and as infrequently as possible.
As a tree grows, its new roots tips, where nutrients are being absorbed, spread out laterally. Expand your watering zone out PAST the tree's canopy as it grows. As the tree grows, continue expanding that water zone. If you have an irrigation system, you need to move the emitters out. If you use a hose, just drag it out further. In any case, water slowly and deeply to ensure water penetration and to leach salts below the root zone.
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